1. Field of the Invention
The present invention is directed to ultrasound transducers for medical applications.
2. Description of the Related Art
Conventional ultrasound transducers used for medical applications, such as High Intensity Focused Ultrasound (HIFU), generate unwanted heat that can affect performance of the transducer. This unwanted heat is due to piezoelectric elements, used therein, having some inefficiency in converting electrical power into acoustic waves. Ceramic piezoelectric elements typically have low thermal conductivity (such as approximately one to two W/mC), which contributes in part to the unwanted heat producing undesirable elevated temperatures.
Some medical and other applications require transducer temperatures to be kept in narrow ranges. For example, when transducers are near or touching biological tissue not intended for treatment, the dosage for this untreated tissue must be held below an equivalent thermal dose of 43 degrees centigrade for 60 minutes. For temperatures above 43 degrees centigrade, equivalent thermal dose is proportional to approximately 2**(T-43), where T is temperature in degrees centigrade. For example, an equivalent thermal dose will also occur at 44 degrees centigrade for approximately 30 minutes and at 50 degrees centigrade for approximately 30 seconds. If a transducer does not come into contact with a patient, generally higher temperatures are permitted, however, temperature levels in excess of 80 or 90 degrees centigrade are most likely to result in damage to the transducer and/or portions of electrical and/or mechanical elements supporting or otherwise associated with the transducer.
For instance, HIFU treatments can involve tens or hundreds of Watts of focused acoustic power resulting in acoustic intensities from 1,000 to 40,000 watts per square centimeter (although typical values are on the order of 2,000 W/cm2); these values can be compared with a few milliwatts per square centimeter for typical diagnostic ultrasound applications. The HIFU treatments can include sound frequencies from one hundred kilohertz to over ten megahertz, with the most common range of 1-10 MHZ.
Conventional attempts at improving performance and lessening other unwanted effects include improving performance so less heat is generated, compensation through electronic controls and/or attempts at removing generated heat. Unfortunately, conventional approaches can lack effectiveness, be cumbersome and/or degrade performance.
A conventional first ultrasound transducer 10, as schematically depicted in FIG. 1 as having elements positioned along an illustrative X-dimension (with a depicted illustrative Y-dimension normal to the X-dimension) to include a rear medium 12, such as air, adjacent to a first piezoelectric element 14, such as a ceramic material, adjacent to a front layer 16. Air is generally useful for the rear medium since it acts as a near perfect reflector in cases where the acoustic impedance of the piezoelectric element 14 is much different than that of air (approximately 0.0004 MRayls). In operation, the front layer 16 is placed adjacent to a front medium 18, such as a tissue of a recipient of ultrasound 20.
The piezoelectric element 14 converts electrical energy into the ultrasound 20, which conducts through the front layer 16 into the front medium 18. The front layer 16 is typically fashioned to help match the acoustical impedance between the piezoelectric element 14 and the front medium 18 for better transfer of the ultrasound 20 from the piezoelectric element to the front medium. For impedance matching, the front layer 16 can be typically as thick as approximately one or more multiples (in particular implementations, odd multiples) of a quarter wavelength of an ultrasound frequency used in operation such as a center operational frequency. The front layer 16 would also have an acoustic impedance to help match impedances of the piezoelectric element 14 (having an acoustic impedance such as approximately 30-35 MRayls) and the front medium 18 (for instance, tissue has an acoustic impedance approximately 1.6 MRayls). The acoustic impedance of single matching layers, such as the front layer 16, can be typically chosen to be within the range of 4 to 8 MRayls. The thermal conductivity of a matching layer in a conventional transducer is often in the range of 1 to 3 W/mC, which is typically the result of loading an epoxy matrix with a higher acoustic impedance and lower acoustic attenuation material such as silicon dioxide or aluminum oxide powder.
Generally, an acoustic impedance for the front layer 16 somewhere between that of the piezoelectric element 14 and that of the front medium 18 is used for acoustic impedance matching of the piezoelectric element and the front medium. Unfortunately, materials used for acoustic impedance matching tend to give conventional matching layers such as the front layer 16 low thermal conductivity. For the front layer 16 between the piezoelectric element 14 having an acoustic impedance Zc and the front medium having an acoustic impedance Zt, the impedance of the front layer 16 can be approximated to be between (ZcZt)1/2 and (ZcZt2)1/3. For example, for a ceramic impedance of 34 MRayls (for the piezoelectric element 14) and tissue at 1.6 MRayls (for the front medium 18), then it would be desirable for a single quarter wave layer to have an acoustic impedance in the range 4-10 Mrayls.
The front layer 16 can also serve to electrically insulate and/or physically protect the piezoelectric element 14 from physical wear or damage. In some applications, the front layer 16 is also shaped to provide an acoustic lens function to focus ultrasound.
A first implementation of the first conventional ultrasound transducer 10 is shown in FIG. 2 and FIG. 3 to include a housing 22 to enclose components enumerated above. With the first implementation, the piezoelectric element 14 and the front layer 16 are formed and optionally adjusted to project the ultrasound 20 to have a focal point 24 located a desired distance into the front medium 18.
As part of the conversion by the piezoelectric element 14 of electrical energy into ultrasound 20, unwanted heat, as mentioned above, is generated by the piezoelectric element. Electronic compensation can be used with the first conventional ultrasound transducer 10 to help partially mitigate effects of the unwanted heat on performance of the first conventional ultrasound transducer.
A second conventional ultrasound transducer 30 is schematically depicted in FIG. 4 to include a thermal heat sink 32 positioned adjacent to the front layer 16 so that in operation the thermal heat sink is adjacent to the front medium 18 as shown. The thermal heat sink 32 is used to remove heat from the vicinity of the piezoelectric element 14 and is fashioned to conduct the ultrasound 20. Unfortunately, in practice the thermal heat sink 32 can be very thick compared with the front layer 16 along the X-dimension of travel of the ultrasound 20 so can also dissipate significant portions of the ultrasound 20 thereby resulting in more heat being generated and reducing efficiency with the transducer performance. The thermal heat sink 32 can also have other operational issues due to its added size and possible use of fluid, such as water, as at least a portion of the thermal mass.
A first implementation of the second conventional ultrasound transducer 30 is shown in FIG. 5 as using a fluid, such as water, for the thermal heat sink 32, which is shown to be contained by a structural appendage 34 and an acoustic membrane 36. Water can serve a dual purpose to cool and also acoustically couple between the second conventional ultrasound transducer 30 and a target. A second implementation of the second conventional ultrasound transducer 30 is shown in FIG. 6 to include a solid, such as a metal, for the thermal heat sink 32. In this second implementation, the structural appendage 34 includes channels 38 for a fluid, such as water, to be passed through to aid in removal of heat. Associated with these first and second implementations of the second conventional ultrasound transducer 30, the use of fluid, additional mass, attenuation of desired ultrasound energy, and positioning of the thermal heat sink 32 and the structural appendage 34 can raise operational issues.